With Auto you mean AGC via setting -10? That leads to a gain a little above 50, which is even higher than the highest manual gain you set manually.
The automatic gain works with a constant signal like TV, but not with the many different and short adsb messages.
To find the „best“ gain value, you can use a python script. Just search this forum for posts by @abcd567, or @obj, who maintains dump1090fa and knows a thing or two about it.
